In the powder coating field of art, polyester resin has been widely used as a base resin. Typical polyester resin is a carboxyl group containing polyester, which is usually coupled with an epoxy compound or resin as a hardening agent.
When the hardening agent is a bifunctional epoxy compound or resin such as aliphatic epoxy resin, bisphenol A type epoxy resin and the like, there are problems such that hardening is fairly poor because of deficiency in the crosslinking degree and processability of the coated product is not so good.
On the other hand, in the case of such hardening agent as novolak type epoxy resin, triglycidyl isocyanurate or other member having much higher functionality (Japanese Patent Publication(unexamined) 69935/77), though the hardening property is excellent, the thus formed coating is poor in flexibility and hence in processability of the formed product.
Also, for the purpose of improving storage stability of powder coating, there has been proposed an addition product of triglycidyl isocyanurate and polybasic acid (Japanese Patent Publication 37343/88).
However, in that technique, when sebacic acid or other polybasic acid having a higher flexibility is used, there results a coating with poor film-hardness, and when terephthalic acid or other polybasic acid having a lower flexibility is selected film-hardness is excellent but there is a problem of poor processability 1,4-cyclohexane dicarboxylic acid possesses intermediate properties of the abovementioned polybasic acids, and therefore, in this case, coating hardness and processability are moderately improved, however, this causes such additional problem that when use as a stored powder coating, there gives only a coated product with less processability.
It is, therefore, an object of the invention to provide a powder coating comprising a carboxyl group containing resin as polyester resin and acryl resin which are most popular in heretofore proposed powder coatings, and an epoxy compound or resin as a hardener, being excellent in storage stability and hardening properties in a wide temperature range of low temperature to high temperature, and capable of producing a coating with excellent processability, even when used as a stored powder coating, and impact strength and the like.